Thunderstorm indices quantify how prone the atmosphere is to triggering storm phenomena.
Even when present, storms might not develop if the initial trigger is missing.
This trigger can be orographic, frontal, or caused by strong surface heating.
Storms can sometimes form even when the indices are unfavorable.
Below is a list of thermodynamic (or thunderstorm) indices:
CAPE (Convective Available Potential Energy)
CAPE measures the total buoyant energy gained by an air parcel as it rises while remaining warmer than its surroundings.
It represents the potential convective energy available in the atmosphere.
CAPE < 400 → No thunderstorms
CAPE 500–1000 → Possible isolated thunderstorms
CAPE 1000–2000 → Thunderstorms fairly likely, possibly strong
CAPE > 2000 → Strong thunderstorms likely, possible tornadoes
CIN (Convective INhibition)
CIN is essentially the opposite of CAPE.
It shows the amount of energy (in joules) preventing vertical convective motion in the lower troposphere.
CIN is a negative value: the air parcel is cooler than its surroundings, so it cannot rise and tends to sink back — a typical temperature inversion scenario.
A high CIN value may also mean that thunderstorm activity could start later in the day, once solar heating removes the stable layer (the CAP).
If other indices are favorable, such situations can produce very violent storms due to explosive updrafts.
When CIN = 0, it means that near-surface layers do not resist cloud formation — provided instability conditions exist.
CIN positive → No inhibition
0 to –20 → Weak inhibition, little to no CAP
–21 to –50 → Moderate inhibition, delayed convection due to CAP
–51 to –99 → Strong inhibition, intense CAP and violent events when it breaks
Below –100 → Very strong inhibition; CAP too strong to break (≈5% chance remains)
CAP (Capping Inversion)
This index quantifies the thermal inversion layer, acting like a lid on a pot.
It’s a warm, dry air layer in the mid-troposphere that suppresses convection while dangerously allowing energy to build up near the surface.
If surface temperature and humidity rise enough, and some mechanism (fronts, orography) forces upward motion, the CAP weakens and convection can suddenly explode — from a nearly stable to an extremely unstable state in minutes.
Strong CAP + favorable setup → Moderate cumulus in the afternoon, clear skies at night
Weak CAP + favorable setup → Early thunderstorm development, less intense
Medium CAP → Ideal for violent isolated storms, as only few updrafts break through
CAP Strength Index (LSI):
LSI > 2 → Convection inhibited, CAP too strong
1 < LSI < 2 → Ideal for strong thunderstorms
LSI < 1 → CAP too weak
LI (Lifted Index)
LI measures atmospheric stability and helps predict thunderstorm intensity.
The more negative the value, the more unstable the air.
LI > 2 → No thunderstorms
LI 0–2 → Possible isolated thunderstorms
LI –2–0 → Thunderstorms fairly likely
LI –4– –2 → Potential for strong thunderstorms
LI < –6 → Strong storms likely, possible tornadoes
SI (Showalter Index)
When SI ≤ +3, it can indicate showers and potential thunderstorm activity.
When SI ≤ –3, it is often linked to strong convection.
SI is similar to LI, and the two are usually analyzed together to assess convective potential.
LI < & SI > → CAP present; convection starts only if CAP breaks
LI > & SI < → Convection starts only with strong lifting
LI < & SI < → Convection very likely
LI > & SI > → Convection unlikely
K (Whiting Index / K Index)
Evaluates instability of an air mass by examining thermal and humidity parameters between 850–500 hPa levels (lower to mid-troposphere).
K < 15 → 0% thunderstorm probability
K 15–20 → 20%
K 21–25 → 20–40%
K 26–30 → 40–60%
K 31–35 → 60–80%
K 36–40 → 80–90%
K > 40 → Over 90%
TT (Total Totals Index)
TT < 44 → No thunderstorms
TT 44–45 → Possible isolated, moderate storms
TT 46–47 → Scattered moderate storms / possible strong storms
TT 48–49 → Scattered moderate / isolated strong storms
TT 50–51 → Scattered strong storms / possible tornadoes
TT 52–55 → Numerous strong storms / tornadoes quite likely
TT > 55 → Numerous strong storms / tornadoes very likely
Precipitable Water
Represents total water vapor in a vertical air column, expressed in mm of water.
Values >20 mm indicate enough humidity for thunderstorm development.
Humidity Index (U)
Not shown in radiosonde data but easy to compute:
U = (1/3) × (RH₈₅₀ + RH₇₀₀ + RH₅₀₀)
where RH = relative humidity at corresponding altitudes (~1500 m, 3000 m, 5000 m).
Thunderstorm probability increases notably when U > 65.
Dew Point
The temperature to which air must be cooled (at constant pressure) to reach saturation and condensation.
In summer, dew point values above 22–23°C near the ground indicate abundant moisture — often fueling severe thunderstorms.
SWEAT (Severe Weather Threat Index)
Considers wind shear and thermodynamic parameters, mainly for tornado forecasting (not necessarily supercells).
SWEAT < 270 → Unfavorable
SWEAT 270–300 → Low tornado chance
SWEAT 300–400 → Moderate tornado potential
SWEAT 400–600 → High tornado potential
SWEAT 601–800 → Very high tornado risk
In the U.S., tornado probability peaks between SWEAT 300–600; higher values are rare.
Statistically, 300 marks the threshold for intense storms and 400 for tornado potential.
In Italy, tornadoes often occur between 250–350, sometimes near 400, and are not always linked to supercells.
SREH (Storm Relative Environmental Helicity)
Measures vertical wind helicity — the tendency for rising air in unstable conditions to rotate, driven by vertical wind shear, upper-level divergence, and surface convergence (possible mesocyclone formation).
A strong indicator for funnels and tornadoes:
SREH quantifies the helical (rotating) component of updrafts between roughly 0–3 km altitude.
SREH ≥ 150 m²/s² → Possible supercells
SREH 150–299 → Moderate chance of weak tornadoes
SREH 330–449 → High chance of strong tornadoes (F2–F3)
SREH > 450 → High chance of violent tornadoes
BRN (Bulk Richardson Number)
Compares convective forcing (CAPE) to vertical wind shear (500 m–6 km).
Useful for identifying supercell potential.
BRN < 10 → Low chance of strong storms
BRN 11–49 → Moderate chance of supercells
BRN 50–100 → High chance of multicell storms / MCCs (supercells also possible)
0–6 KM Shear Vector
Represents wind speed difference between 500 m and 6 km altitude.
Values ≥ 40 knots (≈75 km/h) favor tilted, rotating updrafts.
EHI (Energy Helicity Index)
Used in radiosonde analysis to predict supercell and tornado potential, introduced by R. Davies (1993).
It correlates CAPE and SREH:
EHI = (CAPE × SREH) / 160,000
EHI < 1 → Supercells and tornadoes unlikely
EHI 1–2 → Possible, but weak/short-lived
EHI 2–2.4 → Supercells likely, possible mesocyclonic tornadoes
EHI 2.5–2.9 → Higher mesocyclonic tornado probability
EHI 3–3.9 → Possible F2–F3
EHI > 4 → Possible F4–F5
